Sputtered deposition is a well known technology used to deposit desired materials onto many different items. However, in certain applications, the uniform deposition of sputtered materials across an entire surface of an item is not desired. Many applications require that sputtered materials be deposited in only specific predetermined areas of a single surface. Consequently, the remaining areas of the surface must be covered with a mask, or otherwise shielded, to prevent the deposition of materials onto these areas. A person skilled in the art of sputter deposition will recognize that there exist many types and forms of masks that can be used to shield a desired area, often the form of the mask being dependent on the type and size of the area being shielded. However, a common feature of many masks is that they require a great amount of time and labor to apply and remove from a surface, before and after sputter deposition. If the position of the mask is critical to the eventual operating performance of the item being sputter coated, the time and labor used to meticulously position or align the mask, obviously increases. Similarly, if a mask is placed over an extremely delicate or sensitive surface, great care and, consequently time, must be used when applying and removing the mask from the sensitive surface it covers.
An example of a typical labor intensive, time consuming, prior art masking apparatus and method can be seen by referring to FIG. 1. In FIG. 1, the item on which materials are being sputtered is a photocathode 10, such as the photocathode used in the Generation III image intensifier tube manufactured by ITT Corporation, the assignee herein. The photocathode 10 is generally formed of glass wafer 12 having a concentric raised midregion 14. The raised midregion 14 is coated with a thin sensitive layer of a photoemissive material 16 such as gallium arsenide (GaAs). During sputter deposition, an annular area 18 of chromium is sputtered onto the photocathode 10. The annular area 18 of chromium, coats the region of the photocathode inbetween the raised midregion 14 and a peripheral region 20 on the glass wafer 12. As can be seen, the peripheral region 20 and the center of the raised midregion 14 must be masked prior to the sputter deposition procedure, in order to create the desired annular area 18 of chromium.
To describe the amount of labor and time consumed in sputtering the annular area 18 of chromium onto the above-described photocathode 10, consider the following masking procedure, while referencing FIG. 1. In anticipation of the sputtering procedure, a photocathode 10 is placed into a holding receptacle 22 in a sputter deposition apparatus, such that the raised midregion 14 of the photocathode 10 faces a sputter cathode. To mask the peripheral region 20 of the photocathode 10, preventing the deposition of chromium onto the peripheral region 20, an annular mask 26 is placed over the photocathode 10. To shield the center of the raised midregion 14, a glass masking disk 30 is placed onto the exact center of the midregion 14, such that the glass masking disk 30 contacts the photoemissive material 16. The glass masking disk 30 is centered by an operator utilizing a microscope, referencing the glass masking disk 30 with respect to the outside diameter of the raised midregion 14. If realignment is necessary, an operator must pull the glass masking disk 30 free from the photoemissive material 16 using great care not to damage the underlying photoemissive material 16.
With the glass masking disk 30 properly centered and in place, the retaining fixture 32 is placed over the glass masking disk 30 and the photocathode 10 so as to not disturb the placement of the glass masking disk 30. The retaining fixture 32 includes a contact plate 34, suspended by three arms 36, that abuts against the glass masking disk 30, holding it against the photocathode 10. The retaining fixture 32 is bolted to the sputter deposition apparatus, at points adjacent to the base receptacle 22, using bolts 38 that pass through shaped catch washers 40. Once the bolts 38 are tightened, the alignment of all the elements is again checked by the operator. Similarly, the glass masking disk 30 and photocathode 10 are checked, by the operator, to see if they are properly seated in the base receptacle 22. The operator checks the glass masking disk 30 and the photocathode 10 by pressing the glass masking disk 30 downward with a pair of tweezers at one hundred and twenty degree intervals. If the photocathode 10 moves while the glass masking disk 30 is depressed, the retaining fixture 32 is not properly seated and the entire loading procedure is aborted and begun anew.
The masking technique described in relation to FIG. 1 is highly time consuming and labor intensive. The labor intensive manufacturing process creates many other problems such as, scratch damage to the photoemissive material 16, from the constant repositioning of the glass masking disk 30 by the operator, the misalignment of the glass masking disk 30 by the operator creating a non-concentric deposition, and damage to the photocathode during the removal of the mask. Such errors in manufacturing result in a substantial amount of rejected parts that result in waste and increased manufacturing costs.
Another manufacturing problem, inherent in the described prior art masking method, is that the support arms 36 of the retaining fixture interfere with the sputter deposition of chromium onto the photocathode 10. The shadowing effect of the support arms 36 over the photocathode 10 can create non-uniformity of etch and sputter, which can result in eventual peeling of the deposited chrome and failure in the operation of the photocathode 10.
It is therefore a primary objective of the present invention to set forth a masking device and method wherein a predetermined area on a surface can be masked from sputter deposition in a labor and time effective manner and waste can be substantially reduced by removing operator judgment from the masking procedure.